Iron oxide reduction



June v3, 1952 5. B. WEBB IRON 0x105: REDUCTION Filed July 16, 1948 v ORE AGENT Patented June 3, 1952 IRON OXIDE REDUCTION George B. Webb, Westfield, N. J., assignor to Hydrocarbon Research, Inc., New York, N. Y., a corporation of New Jersey Application July 16, 1948, Serial No. 39,087

Claims.

This invention relates to improvements in the reduction of iron oxide and more particularly to a process for the eflicient utilization of oxygen in generating heat in blast furnaces and for reducing high grade fuel consumption in such furnaces.

In ordinary practice a blast furnace is operated by feeding ore, metallurgical coke and flux such as limestone into the top of the furnace while the preheated air blast is fed into the bottom to react with the coke forming hot carbon monoxide, which reduces the ore and releases the metal which gradually accumulates at the bottom of the furnace. The top or overhead gas containing large quantities of carbon monoxide in addition to carbon dioxide, nitrogen and water vapor is washed for the removal of flue dust. Part of this gas is usually burned to heat the checker-work in stoves for preheating the air blast and the remainder is utilized elsewhere as fuel in operating the plant. The conventional blast furnace process is extremely ineflicient in the sense that it functions as a gas producer manufacturing gaseous fuel from high grade coke which is an expensive raw material in addition to acting as a smelter for the iron ore. Moreover, much of the gas thus formed is not consumed in ore reduction but is either discarded or merely used as fuel where cheaper forms of fuel are readily available.

Various proposals have been made for removing the carbon dioxide and water vapor from the top gas and recycling a portion of the remaining gas, composed largely of carbon monoxide, to utilize more efficiently the carbon content of the gas and decrease the rate of high grade coke consumption. These proposals have involved the use of separate absorption towers and other expensive apparatus which'ofiset any saving realized in coke consumption.

One of the primary objects of this invention is to improve the efficiency of blast furnace operation by recycling overhead gas from which carbon dioxide has been eliminated without the use of absorption apparatus.

Another object of the invention is to effect conversion of carbon dioxide to carbon monoxide by contacting carbon with overhead gas and oxygen at an elevated temperature and to minimize the consumption of oxygen by utilizing the heating value of some of the overhead gas to attain the desired elevated temperature.

Another important object of the invention is to reduce the consumption of high grade coke in blast furnace operation by using a cheaper form of carbon to generate carbon monoxide which is fed to the furnace.

Other objects and advantages of the invention will become apparent from the following description considered in connection with the accompanying drawing which illustrates one form of the invention.

According to the present invention it has been found that substantial economies in the coke requirements of the blast furnace may be effected by contacting a portion of the overhead gas containing carbon dioxide with carbon in a generator at a sufficiently elevated temperature to reduce the carbon dioxide of the gas to carbon monoxide and at the same time introducing oxygen into the generator in sufiicient amount to furnish the heat required for the endothermic reaction. Further, it has been found that if another portion of the overhead gas is burned and the sensible heat of the combustion gases utilized for preheating the portion of the gas,-and preferably also the oxygen, entering the generator, a very substantial saving in oxygen is effected. When these conditions are observed, the oxygen, besides providing heat for the endothermic reduction of the carbon dioxide in the overhead gas passing through the generaton'converts a relatively cheap form of carbon into carbon monoxide and thus further enriches the carbon monoxide content of the gas leaving the generator. The gaseous efiluent from the generator which is rich in carbon monoxide and is at a relatively high temperature is fed to the blast furnace along with oxygen. Preferably, the oxygen is preheated by the combustion of the portion of the overhead gas which does not pass through the generator. Thus, it is seen that the overhead gas of the blast furnace is divided essentially into two portions: one is recycled to the furnace after its content of carbon monoxide has been enriched and the other is consumed to provide heat.

Substantially pure oxygen or oxygen-enriched air (usually not less than about 35% by volume of oxygen) is used both in the generator and in the blast furnace since it is advantageous to avoid the passage of substantial volumes of nitrogen through these reaction units. Oxygen in concentrated form also makes it feasible to operate the generator on a continuous basis.

The recycling of a portion of the overhead gas after enrichment in carbonmonoxide in accordance with this invention decreases'the coke requirement in the furnace. While the coke supplied to the furnace must meet specifications which increase itscost, the generator can opchar, coke-fines and the like.

erate on any cheap fuel such as char, coke fines or breeze, or coke which is unacceptable for use in the blast furnace. Accordingly, a substantial economy in the use of high grade coke in the blast furnace is accomplished.

The recycle of overhead gas by the present method is particularly economical since the oxygen required for addition .to the, generator is curtailed by the preheating of the gas entering the generator. Usually the gas is preheated to a temperature of at least 1300 F. before entering the generator. A sufficient amount of oxygen is then introduced to attain a reaction temperature 1 of about 1800 F. to 2000 F; in the generator.

The use of combustion heat from a portionof V possible with conventional blast furnace practice, byrecycling carbonmonoxide and elimihatin carbon: dioxide in three ways. In the first place, part of the carbon dioxide is eliminated by burningsa portion of the overhead gas containing carbon-dioxide to preheat the recycle portion of; the overhead gas and then discarding this burned- :portion. Secondly, the carbon dioxideiof'the recycle portion is reduced to carbon monoxide at, highitemperature by the use of o yg n and: carbon, and thirdly, the remainder of. the-overhead gas-in excess of that required for 'combustionyor recycle is vented from the system for; 'utilizationxelsewhere.

The'separat iorr and combustion of a portion of the overhead: rg as. materially decreases the thermal load? on 'the generator thereby decreasing the amount :of," oxygen required for reaction heat. I Air'isfmostreconomically used in the combustionofa 'portion3of the. overhead gas for preheating therecycleaportion.

In accordance/with the present invention, a i

very. large proportion of the carbon monoxide used in reducing theioreis developed in a generator whichxutilizesrelatively cheap fuel such as This cheap fuel is utilized initwo princ'ipal ways: (1') for the 1 relativelyinexpe'nsive high temperature reduction of Fthehcarbon dioxide inthe recycle gas,

j and. (2) in the'form'ation of'carbon monoxide directly from the oxygen introduced into the generator.

In generaLaccording to this invention, from I about to 80% of the required carbon is introducedpi'nto the blast furnace-and the remainder (80%.to 20%) is fed-tothe generator. Fre

quently, theidivision is made so th'atto %.1-of the carbon enters the furnace :and the rest goes thine-generator. While it is desirable to supply as muchcarbon as possible byway of I the generator, some ca'r-bon must be charged into the blast furnace to give bed porosity and 1 to facilitateiin other wa-ys the-reduction of the ironore. Inieach "case, the division of carbon depends on, several process'variables including;

he: type of oreiused, the desired *bed depth and porosity in the blast furnace. The selected division of carbon between the furnace and the generator largely determines the proportions of the overhead gas which make up the recycle portion and the fuel portion. For any chosen carbon division, the proportions of overhead gas going to the recycle and fuel portions is readily estimated through heat and material balances. The estimated proportions can be tried and adjusted in a few exploratory runs in the blast furnace to find the optimum proportioning of the overhead gas for use as the recycle and fuel portions. Overhead gas in excess of these two portions which. are used within the process of the invention is withdrawn and utilized elsewhere as a fuel or in any other desired manner.

The oxygen furnished to the blast furnace is in concentrated form, i. e., generally over 35% by volume. Oxygen of at least by volume purity is preferred.

Referring to, the drawing wherein, a flow 'sheet is. represented to illustrate one embodiment, of my process; coke, ore and suitable fiuxin-g. ma terials are introduced, into the top of blast furnace I0 and gradually descend through the furnace while undergoing reduction until molten slag and pig iron are, withdrawn from the lower or crucible portion 'H "through lines t2; and I 3, respectively. The gaseous reactants; hereinafter more fully described, which are fed into the lower. part of furnace It, and the gaseous, reaction products flow countercurrently to thedescending solids and pass as the overhead. gas; composed essentially of carbon monoxide, carbon dioxide and water vapor, through line l'j lto wash tower l5 where a; spray ofwater removes entrained dust and other solid. or condensible impurities. Electrical :precipitators, cyclones and like separators may be employed in place of or in conjunction with wash tower P5. The washed overhead gas leaves tower 15 through line l8 and is madegto. flow by compressor or blower 19.- through two. separate paths. One portion of the overhead gas flows through-:lineill which has branch-lines fl;and" 22-proyided with valves 23 and 24; respectively. Thisi-porltionvof the gas, which; maybe called the recyclegas since it is ultimately returned to furnace I'll,- passes alternately through stoves 25' 'and-26- con1 nected to lines 2| and-"22,, respectively. The other portion of the overhead gas, which may be termed the mer es, flows through; line 211 and alternately through branch lines:28 and-2'9 provided with valves30 and "31, respectively, into stoves 25. and 26. Oxygen, usually the form of air, is supplied through Iiri'es' 32and= 33* to burn the fuel gas alternately e ter 25 and 26, respectively. Thus; during "one interval, says hours, the fuel" stove 25 and raise its temprat of combustion while the recycle gfas isprehjeated by flowing through-stove 2-6 hat'ed' duringfithe preceding interval. In the fsubse'ciuen if" 7 the fuel gasis burned in stoye 2 6 a' nd thefrecycle gas is preheated in stov V j The flue gases resulting from fthe coiiibustion of the fuel portion of the "o'e''rhead;gas;leave stoves 25 and 26 -thrcug'h 'linesi fiid amiss-provide'd with valves ana-anemia. respectively, dis;- charging into -acommonfv'ent line; 2211. The. preheated recycle gas alterhately fiows; from stoves 25 and 2-6-by way-of lines 3 4 fa'n 15*provided with val'ves 38'and 31, respec ely, into line 38 which connected to the genera? tori39t Oxygen required in generator 39 is preferably preheated. In the illustrative flow sheet, the oxygen is preheated in a system similar to that just described for preheating the recycle gas. Oxygen is supplied by line 40 and compressor or blower 4| alternately to lines 42 and 43 which are provided with valves 44 and 45, respectively. Lines 42 and 43 are connected to stoves 46 and 41 from which preheated oxygen may emerge by way of lines 48 and 49 provided with valves 56 and 5|, respectively, and pass through line 52 and valved branch 53 into generator 39.

Part of the fuel gas in line 21 alternately flows through lines 54 and 55 and valves 56 and 51 into stoves 46 and 41 wherein it is burned with air supplied by lines 58 and 59, respectively. The flue gases leave stoves 46 and, 41 through lines 54a and 55a and valves 56a and 51a, respectively, and discharge into vent line 21a.

The preheated oxygen entering generator 39 through line 53 reacts exothermically with the carbon charged thereto through port 60 and thus furnishes heat to the endothermic reaction between the carbon dioxide of the recycle gas fed by line 38 and the carbon in the generator. The gases charged to the generator are proportioned so that the gaseous eflluent leaving through line 5| contains very little, if any, carbon dioxide. Generally speaking, the carbon dioxide content of the generator effluent is maintained below about 10% by volume, preferably below about 5% by volume.

The recycle gas, now substantially free of carbon dioxide and enrichedin the desirable carbon monoxide is passed while still at an elevated temperature through line 6| into the lower section of furnace l0 to take part in the reduction of the iron oxide. Simultaneously, part of the preheated oxygen in line 52 flows through valved branch line 62 into the lower section of furnace H). The oxygen reacts exothermically within the furnace and thus provides heat for the endothermic reduction of iron oxide. The gaseous reactants ascend through the descending mass of ore, coke and flux while reacting and facilitating the reduction of iron oxide. As already stated, the reaction gases leave the furnace l0 through line M as an overhead gas containing appreciable proportions of both carbon monoxide and dioxide. This overhead gas is then treated in the manner hereinabove described. Excess overhead gas which cannot profitably be utilized with the recycle portion or the fuel portion is withdrawn through branch line H; which is connected to line I8 and is provided with valve H. The withdrawn gas has fuel value and may be used in power generation.

While a metallurgical grade of coke is used in the blast furnace I0, a relatively cheap carbonaceous fuel such as char or coke breeze is consumed in generator 39. The ash of the fuel which is charged through port 60 leaves generator 39 at 63.

The advantages of the present invention are more fully appreciated from the following specific example which compares the operational requirements of a blast furnace process employing the principles of the present invention with the requirements of the conventional blast furnace process. The cited gas volumes are based on standard conditions.

For the production of each ton (2000 lbs.) of pig iron, 800 lbs. of coke and the desired fluxing material are introduced along with 3500 lbs, of hematite ore (iron content about 54% by weight) into the top of the blast furnace l0 and gradually descend tothe lower part II of the furnace. The gaseous reactants for each ton of pig iron consist of 10,330 cu. ft. of oxygen (96% by volume purity) supplied by way of line 62 and carbon monoxide-enriched recycle gas supplied by way of line 6| into the lower part H of the furnace to contact the descending solids and thus reduce the iron oxide in the ore. The reaction gases flow upwardly and leave the furnace I0 through line I4 as overhead gas.

The overhead gas amounting to 50,700 cu. ft. per ton of pig iron passes from the top of the blast furnace at a temperature of about 300 F. and is divided as follows: a recycle portion amounting to 12,200 cu. ft. is preheated in the stoves 25 and 26, alternately operated, before entering the generator 39; a second portion amounting to 5,250 cu. ft. is burned and the combustion products used for heating the stoves 25, 26, 46 and 41; and the remainder amounting to- 33,250 cu. ft. is withdrawn from the system and utilized for power generation or other purposes.

The recycle portion of the gas preheated to a temperature of 1300 F. in the stoves enters the lower part of the generator 39 into which 635 lbs. of relatively low grade carbonaceous fuel such as char, coke fines or the like is fed for each ton of pig iron, and passes countercurrent to the carbonaceous fuel charged through port 60. Oxygen (96% by volume purity) in'the amount of 5,540 cu. ft. for each ton of pig iron is also preheated to a temperature of about 1300 F. in stoves 46 and 41 by the combustion products from the second portion of the overhead gas and introduced into the generator 39. The recycle portion of the overhead gas and the oxygen contact the carbon in the generator raising the reaction temperature to about 1800-2000 F., the carbon dioxide in the recycle gas is reduced to carbon monoxide, the oxygen forms additional carbon monoxide by reaction with the free carbon, and the gas thus enriched in carbon monoxide flows from the generator by way of line 6| and enters furnace III at a temperature of about 1300? F. The oxygen, previously mentioned as'being supplied to furnace I0, is also preheated to a temperature of about 1300 F. by combustion of the second portion of the overhead gas in the stoves 46 and 41. The pig iron and slag are tapped from the furnace at a temperature of about 2500 F.

With the conventional blast furnace process, using the same iron oxide ore of the foregoing illustrative example of the invention, each ton of pig iron produced requires a high grade coke input to the blast furnace of 1590 lbs. The reduction of the iron oxide requires for each ton of pig iron a blast of 75,700 cu. ft. of air producing an overhead gas of 103,400 cu. ft. This overhead gas is ordinarily divided into two portions, one portion amounting to 22,400 cu. ft. being used as fuel in stoves for preheating the air blast and the remaining portion of 81,000 cu. ft. being drawn off from the system for use as fuel or in any desired manner.

It will be appreciated that the process of the invention, as illustrated in the specific example, has enabled a 50% decrease in the amount of high grade coke required in the conventional operation of the blast furnace. The high grade coke requirements may be further reduced by increasing the input of low grade fuel to the generator and suitably adjusting the conditions of operation. Thus, a very substantial proportion of the fuel requirement of the furnace is obtained; through the use of -l'ow gra'd'e fuel in the generator. Furthermore, it is evident from the example that lthefioverall' fuel' requirements of the furnace, when operated in accordance with this invention, are substantially decreasedin that a total of 1435. lbs. of carbonaceous fuel is con sumed as against 1590 lbs. in conventional operation'. A further advantage is that the volume of gases preheated by the stoves is much less and therefore smaller stove and'blower capacities are required. 'The overhead gas available for fuel from the process ofthe invention, being deficient in nitrogen, is of substantially higher heating value than the overheadfgas from the conventional blast furnace process and is suitable for use as'fuel in open hearth and similar steel refining units. 7 7

'It will be clear to those skilled in the art that because of the decreased quantity of carbon charged into the reducing furnace and of the oxygen-rich gassuppliedihereto in place of an ordinary air blast, the. present. invention permits theuse of reducing'i'urnaces of decreasedlheight. Decreased furnace height means that the .porosity of the reactionbed canfbe lower than that customarily maintained in blast furnaces and, in turn, this means that the carbonaceous solid or coke introduced into the reducing .furnace can have a lower crush resistance. Accordingly, the invention'affordsthe opportunity not only cidecreasing the consumption o'fcoke or. like carbonaceous solid but also of utilizing cakes or inferior quality (relatively llowstrengith or crush resistance) Various modifications 6f the invention will ocour to those skilled in the art up'on' consideration' of the foregoing disclosure, without departing'f'rom the spirit or scope thereof. According- 1y, only such limitations shouldibeimposed' as are indicatedbyjth appended'claims.

Iclaim': r

'1. Intheprocessiof operatingashaft-type furnace to reduce iron oxide wherein .a gaseous. ef-

fiuentfroni.saidlfurnacecontains essentially carbon monoxide and carbon dioxide and 'i recycled .to 'said furnace after treatment to-decrase substantially the carbon dioxide content of-said gaseous 'effluent'fthe',improvernent which comprisesburning'a portion of said gaseous efiliien't to preheat a second portion thereofreacting solely the 3 preheate'dj portion of said gaseous effluent and oxygenof at leastabout' 90% "byrvolume purity with coke at a temperature ofat least'1'800F. -to

produce 'a' gaseous stream enriched incarbon monoxide, and passing said gaseous. stream' an'd additional oxygen of" at least aboutf90% by vol 'ume purity through said furnace "to effect red-tic tion .of iron oxide. v

2. In the process offoperatinga blastfurnace to'redu ceiron oxidewlrerein an overhead gascontainingessen'tially carbon monoxide and carbon dioxide is'recycled to said 'furnaceaftertreatrnent to decrease substantially theparbon'dioxide-content ofsaid over'head gas,"the improvement-which comprises burning a portion of sa idfoverhead gas to preheat asecond portion thereof and oxygen of at least about by volume purity to a temper'ature of at least 1300" F., reacting solely the preheated portion of said overhead gas and the preheated oxygen with cake at a temperature of atleast 1800 F. to produce agaseous stream enriched in carbon monoxide, and introducing said gaseous stream and additional oxygen of at least about 90% by: volume purity into the lower part of-said'furnace to efifect reduction of iron oxide. 3'. In the process of operating a blast furnace to reduce iron oxide wherein an overhead gas containing essentially carbon monoxide and carbon dioxide is recycled to said furnace after treatment to decrease substantially the carbon dioxide con tent of said overhead gas, the improvement which comprises burning a portion ofsaid overhead gas to preheat a second portion thereof and a gaseous stream containing at least about 90% by volume of oxygen to a temperature of at least 1300 F., reacting solely the preheated portion of said overhead'gas and the preheated oxygen-containing stream with coke by upward passage through a downwardly moving bed ofisaid coke to produce a gaseous stream enriched in carbon monoxide, and introducing the carbon monoxide-enriched stream and a second gaseous stream-containing at least about 90% by volume of oxygen into the lower part of said furnace to effect reduction of iron oxide.

4'. The process of claim 2 wherein the gaseous stream enriched in carbon monoxide has a carbon dioxide content below about5f% by volume. 5. The process of claim 3' wherein the second gaseous stream containing atleast about 90% by volume of oxygen is preheated to a temperature of at least 1300* F. before introduction into the lower part of the blast furnace.

GEORGE B. WEBB.

REFERENCES CITED The following references are of record in the file of this patent.

UNITED STATES PATENTS" Number 7 Name Date 590,925 Westman Sept. .28, 1897 891,248 Gronwall .June23, 1908 1;795,829 Brassertet al.- Mar. '10, 1931 .1','803,68.6 Andrews May 5,1931

1,842,609 Hillhouse \Jan. 26,1932

1,850,009 Gronwall'aeta'l Mar. 15, 1932 1,984,727 Brown Dec. 18; 1934 FOREIGN PATENTS- Number Country Date 369393 Great Britain Mar. 24, 1932 521;?15 Great Britain May 21, 1940 OTHER REFERENCES I 'Ser. No; 360,935; SchwierKA. P. 0:), published May 4,1943. 7 

2. IN THE PROCESS OF OPERATING A BLAST FURNACE TO REDUCE IRON OXIDE WHEREIN AN OVERHEAD GAS CONTAINING ESSENTIALLY CARBON MONOXIDE AND CARBON DIOXIDE IS RECYCLED TO SAID FURNACE AFTER TREATMENT TO DECREASE SUBSTANTIALLY THE CARBON DIOXIDE CONTENT OF SAID OVERHEAD GAS, THE IMPROVEMENT WHICH COMPRISES BURNING A PORTION OF SAID OVERHEAD GAS TO PREHEAT A SECOND PORTION THEROF AND OXYGEN OF AT LEAST ABOUT 90% BY VOLUME PURITY TO A TEMPERTURE OF AT LEAST 1300* F., REACTING SOLELY THE PREHEATED PORTION OF SAID OVERHEAD GAS AND THE PREHEATED OXYGEN WITH COKE AT A TEMPERATURE OF AT LEAST 1800* F. TO PRODUCE A GASEOUS STREAM ENRICHED IN CARBON MONOXIDE, AND INTRODUCING SAID GASEOUS STREAM AND ADDITIONAL OXYGEN OF AT LEAST ABOUT 90% BY VOLUME PURITY INTO THE LOWER PART OF SAID FURNACE TO EFFECT REDUCTION OF IRON OXIDE. 